A glass tube and a glass rod are to be understood to mean herein a substrate tube or tube to be coated and a rod, which consist of synthetically manufactured amorphous silica or of amorphous silica manufactured from quartz crystals by melting (fused silica, quartz glass) with the tube material optionally being doped, or which consist of both synthetically manufactured amorphous silica and of amorphous silica manufactured from quartz crystals by melting (fused silica, quartz glass), with the tube material optionally being doped. The deposited glass consists of synthetically manufactured amorphous silica which is possibly doped.
The manufacture of optical fibers or optical waveguides according to the above-mentioned method is known from, inter alia, U.S. Pat. Nos. Re. 30,635 and 4,314,833, the version in which the glass is simultaneously deposited on a glass rod which is arranged inside the glass tube, being known from U.S. Pat. No. Re. 30,635. The present method of manufacturing except for said version in practice is referred to as "nonisothermal plasma-CVD method" (nonisothermal PCVD method, in which P=plasma and CVD=chemical vapor deposition=reactive deposition from the gas phase). In this method, glass layers are directly deposited from the gas phase on the inner wall of the glass tube (heterogeneous reaction). In this way, the formation of glass soot in the gas phase is avoided. This is described in more detail, in particular, in U.S. Pat. No. 4,314,833.
Using the PCVD method, graded index fibers as well as stepped index fibers or fibers having another refractive index profile can be manufactured, with quantities of glass being deposited which correspond to the relevant fiber construction.
The PCVD method is distinguished by its deposition yields of almost 100% and the high uniformity of the deposited glass layers along the deposition zone. However, when the total gas flow exceeds a certain value, for example, because higher deposition rates are required, deviations from the uniformity of the layers along the tube axis occur which make further processing of the starting and end regions of the coated glass tube into optical fibers impossible. Such regions are called tapers. They cause a loss of yield.
It is disclosed in DE-OS 34 45 239 (which corresponds substantially to U.S. Pat. No. 4,741,747 which is commonly assigned herewith) and DE-OS 35 25 979 that by slowing down and accelerating the plasma movement and/or by changing the microwave power supplied in the front portion of the taper, the part of the coated tube suitable for further processing can be enlarged. Both measures have in common that for reasons of process requirements a reduction of the geometrical taper leads to the formation of optical tapers. In this respect, a geometrical taper is to be understood to mean a change in the coating thickness per unit of length and an optical taper is to be understood to mean a change in the refractive index profile along the tube.